Module 9

Question 1

Define a redox reaction and determine if it will go forward at standard state given appropriate data.

Answer 1

Reverse: Eh° < 0 & ΔG°R > 0

forward: Eh° > 0 & ΔG°R < 0

Positive E = reaction is spontaneous (energy-producing)!

Question 2

Interpret an Eh-pH diagram for any system. Determine which chemical species is thermodynamically favourable for different conditions

Answer 2

-plot stable phases of electrochemically important species in an aqueous solution

Top: O2 stability range
H2O is oxidized to O2

Line: p(O2)=1
middle: H2O stability range

Line: p(H2)=1
Bottom: H2 stability range
H2O is reduced to H2

 Mine water
-> acidic from oxidation of sulfide minerals
-high Eh assuming in equilibrium with atmospheric O2

 Groundwater
-> isolated from
atmosphere,
-Eh drop over time as O2 is consumed by microbes
 May have higher pH due to dissolution of carbonate minerals

 Higher O2 -> oxidizing environment, higher Eh

Question 3

Determine the equations for lines on Eh-pH diagrams given appropriate thermodynamic data

Answer 3

 Lines between species indicate areas where activities are equal

Three possible types of lines on Eh-pH Diagram:

 Vertical line: pH reaction, no change in oxidation state (e.g. acid dissociation, use pKa), 2 species equal

 Horizontal line: Redox reaction with no H+ involved

 Sloped line: Redox reaction involving H+, ph is variable, different v depending on ph

Question 4

Define a redox cascade and order the reactions if given appropriate thermodynamic data.
Give examples where redox cascades occur in the environment. Explain why redox
cascades can occur even when environments are open to the atmosphere

Answer 4

-O2 is used up
-anaerobic respiration yielding most energy will occur first

 EAs are used up in order (often, pretty much completely!), from most energy-yielding to least

-sequence of respiration reactions
that is predictable with thermodynamics

Anytime O2 in water is used up and not replenished (e.g,. a closed system) and there is available organic carbon

Examples:
 Porewater in sediments
 Hypolimnion (bottom) of stratified lake
 Groundwaters that run out of O2
 E.g. groundwater with high organic matter (this doesn’t always happen), could be natural or organic contamination

Ocean water is well oxygenated
so top of sediments is oxic

cascade can happen in open systems at night

rate &laquo_space;net O2 uptake rate
-O2 decreases over time
-at night, no photosynthesis

Question 5

Describe how redox reactions can be used to clean up contaminants, with an example

Answer 5

-groundwater is oxic
-low organic carbon
-respiration is low

dense non-aqueous phase liquid
-Spilled DNAPL is an
organic compound

-Microbes DNAPL as electron acceptor, first reacts with O2 (front of plume)

• When O2 is used up, move
  on to NO3-, etc.

denitrification occurs when O2 is low
So will persist in oxic aquifers.
-runs out of O2 (high organic C and high aerobic respiration), denitrification will occur

Permeable Reactive Barriers (PRBs)
-Trenches dug into aquifer
-permeable substance
-promotes a redox reaction to use up a
contaminant

 O2 is used up with aerobic
respiration
 Denitrification then proceeds
and uses up NO3-

Question 6

Explain how salinity affects redox reaction rate and give some examples

Answer 6

Increasing salinity in water helps transfer electrons (increases electrical conductivity)
- salinity makes redox faster

 Examples?
 Car rust accelerated by the use of road salt

 High salinity can increase Pb2+ leaching from Pb in pipes and solder

Question 7

redox

Answer 7

LEO the lion says GER:
Loss of Electrons: Oxidation;
Gain of Electrons: Reduction

Question 8

oxidation states

Answer 8

H is +1
 Unless it’s in a hydride (e.g., LiH), in which case it’s -1

 Oxidation state of O is -2
 Unless it’s in a peroxide, in which case it’s -1 (e.g., H2O2)

diatomic is 0 (H2,O2, etc)

• pure elemental C(s) is 0